1. Field of the Invention
This invention relates to an apparatus and method for controlling a welding parameter in an arc welding system. More particularly, the invention relates to an apparatus and method for adjusting the frequency or some other parameter of pulses applied to an electrode of an arc welding system in response to changes in the arc voltage, and for returning the pulse frequency or other pulsation parameter to a desired value by varying a further parameter. In an illustrated example of the invention, the arc welding system is a consumable electrode arc welding system, the pulsation parameter may be the pulse frequency, and the further parameter that is varied to return the pulsation parameter to the desired value is the distance between a workpiece and the consumable electrode or an electrical connection element for the consumable electrode, such as a contact tip. The invention may also have applicability to pulsed arc welding systems which use non-consumable electrodes.
2. Description of Related Art
It is well-known to control the arc in an arc welding system by varying the power supplied to an electrode to compensate for changes in the arc voltage as a result of changes in the distance between the workpiece and the electrode or the contact tip that supplies power to the electrode. Such changes occur because of variations in workpiece contour as the torch moves along a weld path, or misalignment of torch travel relative to the workpiece, and must be rapidly compensated in order to ensure good weld quality.
One way to compensate for arc voltage deviations caused by changes in the distance between workpiece and the electrode or contact tip is to mechanically move the electrode or contact tip in response to changes in the arc voltage. In the case of a consumable electrode system, it is the contact tip that is moved, the electrode being arranged in sliding engagement with the contact tip and fed at a constant rate. An example of such a proximity controller is described in U.S. Pat. No. 4,631,385 (Rothermel)
Another way to compensate for variations in the distance between the contact tip and the workpiece is to adjust an electrical parameter of the power supplied to the electrode, such as the average current, or in the case of pulsed currents, the peak and/or base amplitudes, width, or shape of waveforms supplied to the contact tip in response to changes in the arc voltage. Examples of current control based on the arc voltage are found in U.S. Pat. No. 3,614,377 Stearns), U.S. Pat. No. 3,896,287 (Cook), and U.S. Pat. No. 3,904,846 (Risberg), with the Stearns and Cook patents being directed to pulse width modulation control of constant current arc welding power supplies, and the Risberg patent disclosing adaptive control of both the pulse width and interval of the arc welding current. Such systems are effective in controlling the overall burn off rate, but only so long as the adjusted parameter is kept within relatively narrow limits since the weld quality, particularly in pulsed arc welding systems, is highly sensitive to the current amplitude and waveform. For example, too low a current or too short a pulse width will cause globular and intermittent metal transfer with attendant spatter. Detailed descriptions of the considerations involved in selecting a suitable waveform are described in a number of the U.S. patents cited herein.
The present invention is particularly directed to an arc welding control system of the type in which power to the contact tip is in the form of pulses and the pulse frequency is controlled based on changes in voltage between the contact tip and the workpiece. Such systems are known, for example, from U.S. Pat. No. 4,409,465 (Yamamoto), U.S. Pat. No. 4,427,874 (Tabata et al.), U.S. Pat. No. 4,620,082 (Graville), and U.S. Pat. No. 4,758,707 (Ogilvie). However, as in other conventional current or pulse control systems, the conventional pulse frequency controls are limited so a relative narrow adjustment range and cannot be used where the variations in arc voltage are likely to cause the adjusted parameter to deviate beyond an acceptable range. Since the pulse frequency is a critical parameter in pulsed consumable electrode arc welding systems, variation in the pulse frequency based on arc voltage has not been widely utilized, despite the simplicity and accuracy of frequency controllers in general.
The present invention solves this problem by adding a control loop which returns the adjusted parameter to a desired value by adjusting the contact tip-to-workpiece distance. This action also restores the contact tip-to-workpiece distance back to its original or intended value, which further guarantees that gas coverage and electrode stick-out (i.e., the distance between the end of the contact tip and the arc) remains constant. As will be discussed below, the use of an additional, proximity based control loop to return a deviating parameter to its original or desired value is known from U.S. Pat. No. 4,794,232 (Kimbrough et al.).
Before discussing the Kimbrough et al. patent, the conventional pulse frequency control system will be described. FIG. 1, shows the basic elements of a conventional arc welding system of the type which uses a torch to which is supplied a consumable electrode as the source of welding material. During welding, a voltage is applied between the contact tip 1 and a workpiece 2, and the consumable electrode 3 is continuously fed from a spool 4 towards the workpiece by rollers 5 driven by a motor 6 at a rate corresponding to the rate that material is burned off the tip of the electrode 3 by the applied voltage, so that a constantly controlled amount of material is deposited as the torch is moved along the weld. A feed rate controller 7 is used to regulate the feed rate in accordance with commands entered manually or by computer via input 8. Power to the electrode is provided by a weld power source 9 that delivers electrical power to the electrode via the sliding contact tip 1.
In this type of consumable electrode arc welding system, the arc voltage is a function of the distance between the contact tip and the workpiece, while the overall rate of burn off of the consumable electrode is determined by the applied voltage and current, i.e., by the power applied to the electrode. In order to compensate for increases or decreases in the torch-to-work distance and/or arc length, and thereby maintain a constant burn off rate, some conventional pulsed arc welding systems proportionally increase or decrease the power supplied to the contact tip by varying a nominal pulse frequency input 10 based on feedback from a voltage sensor 11. Adder 12 compares the voltage sensor feedback with the desired arc voltage input 13 and the resulting difference signal is processed by processor 14 to obtain a compensation signal. The feedback signal may be representative of a peak voltage during the pulse, a background voltage between pulses, an average voltage, or an instantaneous voltage, taken at any point in the pulse cycle or over multiple cycles. The resulting frequency command is then supplied to a waveform generator 15 which generates the desired waveform, which may optionally be based on a stored or preset pulse profile data input 16, at the commanded frequency to obtain a control signal for the weld power source. If the arc voltage is less than the desired voltage, the feedback loop automatically increases the pulsation frequency, causing the average current and electrode consumption rate to increase, until the arc voltage equals the commanded value. Conversely, if the arc voltage is greater than the desired voltage, the loop automatically decreases the pulsation frequency, causing the consumption rate to decrease, until the arc voltage again equals the commanded value.
The purpose of pulsing the power supply to the electrode is to facilitate control over the size of the molten puddle by controlling the average weld power via pulse frequency and feed rate adjustments. In addition, the power delivered during each pulse controls and maintains the uniform detachment of electrode material, thereby preventing globular and intermittent metal transfer with its attendant weld spatter. The pulse repetition rate or frequency dominates in controlling average power, and therefore the pulsation frequency also dominates the electrode consumption rate. As a result, the pulsation frequency must be closely coordinated with the electrode feed rate, because equilibrium requires that the consumption rate equal the feed rate.
As indicated above, the problem with the conventional frequency based control system is that there is a limited range over which the frequency can be varied. The present invention solves this problem by adding a secondary control loop that tends to return the frequency to its original value by adjusting another arc length related parameter that has the effect of countering changes in the arc voltage. In the preferred embodiment of the invention, for example, the parameter varied is the torch-to-workpiece distance or proximity, although it is also within the scope of the invention to control other arc length and/or position related parameters such as weld seam tracking and scanning parameters involved in scanning of the torch relative to the workpiece. Scanning arrangements to which the principles of the invention may be applied include those disclosed in U.S. Pat. No. 3,818,176 (Brown), U.S. Pat. No. 3,832,522 (Arikawa), U.S. Pat. No. 4,019,016 (Friedman et al.), and U.S. Pat. No. 4,531,192 (Cook).
While the use of proximity feedback control to return a deviant parameter to its desired or original value is disclosed in U.S. Pat. No. 4,794,232 (Kimbrough et al.), the Kimbrough et al. patent concerns systems in which torch proximity is adjusted or controlled in order to return the electrode feed rate or average current to its desired or original value. This is in contrast to the system of the present invention, in which the secondary feedback loop returns pulse frequency to a desired value using a proximity controller to counter pulse frequency variations resulting from changes in the contact-tip voltage (which is often loosely referred to as the arc voltage, even though the arc voltage does not include the voltage drop from the contact-tip to the consumable electrode, and along the consumable electrode to the arc).
In the Kimbrough et al. systems involving pulsation, the width and spacing of the pulses is constantly controlled to supply a predetermined amount of power to the consumable electrode during each cycle, with the width of the pulses being controlled based on the total pulse energy supplied to the electrode, and the interval between pulses being adjusted to control either the average voltage or average current over the previous peak and current base period. This is accomplished by calculating the total energy delivered at peak amps based on the current supplied to the contact tip and the arc voltage so that when the total energy equals a desired pulse energy the system switches to a base current. The duration of the base current is determined either by the average arc voltage over the preceding peak period and present base period or the average current during the preceding peak and current base period.
The three control systems of Kimbrough et al. that include both pulsation and proximity controls have, at least, the disadvantage of being more complicated than the system of the present invention in that they employ three or four control loops, not counting a possible fourth or fifth loop to regulate pulsation current. All three of these systems use a first control loop to regulate pulse energy, as previously described. In addition, one of the systems employs a second loop that adjusts the interval between pulses to regulate average arc current to a desired value, a third loop that adjusts the wire feed rate to maintain or regulate the electrode voltage, and a fourth loop that adjusts proximity to maintain or regulate the wire feed rate to a desired value.
The second and third of the three pulsation systems disclosed by Kimbrough et al. employ a second control loop that adjusts the interval between pulses to regulate average voltage to a desired value. The second system further employs a third loop that adjusts the wire feed rate to maintain or regulate the average arc current, and a fourth loop that adjusts torch proximity to maintain or regulate the wire feed rate to a desired value. Finally, the third system further employs a third loop that adjusts or modulates torch proximity as a way of maintaining or regulating the current to a desired value, not counting a likely fourth loop used to maintain constant feed rate.
Nowhere does Kimbrough et al. use pulse width, pulse amplitude, background amplitude between pulses, or frequency as a control variable in a control loop to regulate arc voltage, much less use only one additional loop to return this control variable to its desired or original value.
In the present invention, the feedback signal for the second loop (e.g., pulsation frequency) can be determined directly from arc current or arc voltage measurements. However, in a further improvement of the present invention, a signal representing the control variable for the first loop is used directly as the feed back signal for the second loop, making direct measurement unnecessary. With this improvement, only one direct measurement is needed, namely contact tip-to-workpiece voltage, in order to control both contact-tip voltage and torch proximity. This is in contrast to the systems of Kimbrough et al. in that each of the Kimbrough et al. systems requires multiple parameter measurements.
The Kimbrough et al. systems also use a recovery condition that averages either voltage or current over only one cycle on a cycle-by-cycle basis, i.e., the average value for each cycle is used to determine the pulse spacing for that particular cycle. Since electric arcs are inherently noisy, the pulse spacing, and therefore the arc length, varies from cycle-to-cycle in response to arc induced noise. In contrast, the present invention is not limited to cycle-by-cycle averaging. Instead, the averaging can be, and is preferably, performed over more than one circle.
In the present invention, it is also preferable to use a plurality of digital signal processors (DSPs) because such processors are ideally suited for performing digital filtering and control, which serve to enhance the signal-to-noise ratio, arc length stability, and control capabilities. In addition, each time the arc voltage control loop digital signal processor updates the pulse parameter control variable or variables, it sends a value or values corresponding to the updated parameter or parameters directly to the second control loop digital signal processor for use as its feedback signal.
Although a number of prior patents refer simply to measurement of the xe2x80x9carc voltage,xe2x80x9d those skilled in the art will appreciate that the arc voltage is technically only the voltage difference between the end of the electrode and the workpiece over which the arc extends, and not the voltage from the contact tip to the workpiece. The contact tip-to-workpiece voltage differs from the arc voltage due to voltage drops from the contact-tip to the consumable electrode and along the electrode between the end of the contact tip and the arc. Therefore, depending on whether the welding system is a consumable electrode or fixed electrode arc welding system, the voltage measured for the purpose of detecting changes in arc length may either be the electrode-to-workpiece or the contact tip-to-workpiece voltage. For purposes of providing distance related data as inputs to the first feedback loop of the present invention, the electrode-to-workpiece and contact tip-to-workpiece voltages may be considered to be equivalent.
It is accordingly a first objective of the invention to provide an apparatus and method for improving the stability and accuracy of a pulse frequency (or other pulsation parameter or parameters) controlled arc welding control system by controlling a second parameter in order to bring the pulse frequency (or other pulsation parameter or parameters) back to a commanded value.
It is a second objective of the invention to provide a dual loop control apparatus and method for an arc welding system in which a proximity controller is used to return an adjusted parameter to its desired value without necessarily requiring more than one parameter measurement.
It is a further objective of the invention to provide an apparatus and method of controlling an arc welding system of the type in which the pulse frequency, or other pulsation parameter or parameters, is adjusted to compensate for deviations in a first parameter related to the distance of the contact tip from the workpiece, and in which a second parameter is controlled in order to bring the pulse frequency or other pulsation parameter or parameters back to a commanded or initial value, thereby adding a second feedback loop to the first in order to optimize all parameters having a significant effect on weld quality.
These objectives of the invention are achieved, in accordance with the principles of a preferred embodiment of the invention, by providing an arc welding control method and apparatus in which the conventional weld power source control loop is modified by adding an additional feedback loop employing an electromechanical controller, such as a proximity controller. The faster acting weld power source control loop directly adjusts a pulsation parameter such as the pulse width, pulse amplitude, background amplitude between pulses, or pulse frequency to regulate arc voltage, while the slower control loop adjusts a further parameter to regulate the pulsation parameter and return it to a desired value via the voltage controller""s interaction with the arc process. The feedback value for the slower loop may either be the command signal for the pulsation parameter used by the faster loop, or in an existing system in which a command signal for the pulsation parameter is not available, a value that is determined by sensing and processing the actual arc current or arc voltage.
In an illustrated implementation of the preferred embodiment of the invention, the additional or secondary feedback loop adjusts the torch or contact tip-to-workpiece distance using a proximity control motor. In the case of a pulse frequency controller, for example, when the torch-to-work distance begins to increase due to misalignment or changes in workpiece contour encountered as the torch moves along the weld path, the weld power source voltage control loop will decrease the pulse frequency to keep the voltage equal to the voltage command. The slower proximity control will in turn decrease the torch-to-work distance until the voltage control loop returns the frequency to its original value, which corresponds to the original torch-to-work distance, all other parameters remaining constant. Conversely, if the torch-to-work distance attempts to decrease, the weld power source voltage control loop will increase the pulse frequency to hold the voltage equal to the voltage command. The proximity controller will in turn increase the torch-to-work distance until the voltage control loop returns the frequency to its initial value which again corresponds to the original torch-to-work distance.
Although it is possible to use a preset or externally input reference frequency, the preferred embodiment of the invention acquires the reference by locking the position of the torch relative to the workpiece (or using a fixed scanning path) and storing the frequency that results from operation of the primary control loop for use as a reference frequency.
It is also within the scope of the invention to use the voltage control loop in combination with electromechanical or magnetic elements for moving or scanning the torch and/or the arc across the surface of the workpiece in order to gather and store position-related frequency information. When correlated to the motion pattern, this position related information can be further processed and used to control such things as torch proximity, weld-seam tracking, automatic cross-seam oscillation width adjustment, and so forth.
The preferred method of compensating for changes in the arc voltage between the workpiece and the contact tip of a consumable electrode arc welding system or the electrode of other types of arc welding systems includes the steps of supplying a current pulse train having a desired pulsation parameter to the contact tip or electrode in order to establish an arc between the electrode and the workpiece, measuring an electrical parameter of the arc such as the arc voltage and varying the pulsation parameter in response to changes in the electrical parameter, and adjusting a parameter related to electrode or contact tip position in order to cause the pulsation parameter to return to the desired value. Adjustment of the parameter related to electrode or contact tip position is preferably carried by the steps of initially locking the position of the torch, establishing an arc, waiting for the system and pulsation parameter to settle, and storing the resulting pulsation parameter as a reference pulsation parameter. Once the reference pulsation parameter is stored, the position of the torch is unlocked and as the pulsation parameter is subsequently adjusted, it is compared with the stored reference to provide a difference signal which is used as the basis for adjustment of the contact tip or electrode position (conveniently referred to as the torch position) until the torch position and/or other parameters are reset or adjusted by an external input.